Principle of a Battery

Galvanic corrosion works like a battery built from two electrodes  

This cell produces electricity while consuming zinc that precipitates as zinc hydroxide 

Three conditions must be met simultaneously for a cell to function  

In galvanic corrosion, the cathodic reactions (on the metal which does not undergo corrosion) depend on the medium  

If one of these three conditions is not met, for example, if the electrical connection is interrupted, the cell will not produce a current, and the oxidation at the anode will not take place (nor the reduction at the cathode). 

---